Field of the Invention
The present invention relates to a digital key telephone system (DKTS) or a simplified telephone switchboard (referred to as a digital key telephone system generically, hereinafter), and more specifically to a digital key telephone system which can increase the number of extension telephone sets by interconnecting a plurality of main installations (i.e. systems).
The method of increasing the number of extension telephone sets in a digital key telephone system can roughly be divided into two, building block and system interconnection, methods. In the building block method, a number of extension modules called extension racks are stacked on the main installation called a basic rack as building blocks in order to increase the number of extension telephone sets connectable to the main installation. In the system interconnection method, a plurality of the main installations arranged at intervals of 300 m, for instance are interconnected by some transmission lines in order to implement some simple functions such as extension-to-extension calls between two main installations (racks) or transfer of incoming calls.
The present invention relates to a digital key telephone system applicable to the latter (system interconnection) method.
The prior-art digital key telephone system applicable to the intercommunication method is generally constructed as follows: Each of plural main installations interconnected to each other is provided with an analog exchange office line interface unit to which incoming call lines are connected to transmit/receive analog signals between the main installation and the telephone exchange office and with an analog standard telephone set interface unit to which standard telephone sets (extension terminals) are connected to transmit/receive analog signals between the main installation and the standard telephone sets. For instance, when the two main installations are interconnected to each other, the analog exchange office line interface unit provided for the first main installation is connected through an analog line to the analog standard telephone set interface unit provided for the second main installation, while the analog standard telephone set interface unit provided for the first main installation is connected through another analog line to the analog exchange office line interface unit provided for the second main installation. In other words, the two main installations are interconnected crosswise to each other by two analog lines. Therefore, extension-to-extension calls or incoming call transfers are implemented between one main installation (system) and the other main installation (system) through the analog interfaces of the main installations and the analog transmission lines interconnecting the two installations.
In the prior-art digital key telephone system as described above, speech signals are converted into digital data and then transmitted or received via a channel in a time division multiplexing method between the two main installations. On the other hand, however, since the transmission line network outside the main installations and the telephone sets are of analog type, the line network and the telephone sets are connected to the main installation via an analog interface unit. Further, the two systems (main installations) are also interconnected to each other via the analog interface units. Therefore, when non-telephone terminal devices (e.g. data terminal devices) are connected to the main installation, it has been necessary to provide a MODEM (modulator-demodulator) in order to implement extension-to-extension communication of non-telephone terminal devices connected to two different systems.
In addition, since the two main installations are crosswise connected to each other as described above, there exists another problem in that the number of wires increases when the number of main installations interconnected to each other increases.